Preparation of steel for glassing and resultant article



y 10, 1956 J. H. HEALY ETAL 2,754,222

PREPAR TION OF STEEL FOR GLASSING AND RESULTANT ARTICLE Filed Nov. 9,1953 2 Sheets$heet l INVENTORJ.

James H. Healy BY James D.5ulliucm ATTORNEYS.

July 10, 1956 J. H. HEALY ETAL 2,754,222

PREPARATION OF STEEL FOR GLASSING AND RESULTANT ARTICLE Filed NOV. 9,1953 2 Sheets-Sheet 2 %FIG.4.

! FIG. 7.

INVENTORS:

James H.Hecz lg BY James D.5ullzucm gnaw $44,

ATTORNEYS.

United States Patent PREPARATION OF STEEL FOR GLASSIN G AND RESULTANTARTICLE James H. Healy and James D. Sullivan, Milwaukee, Wis., assignorsto A. 0. Smith Corporation, Milwaukee, Wis, a corporation of New YorkApplication November 9, 1953, Serial No. 391,024 7 Claims. (Cl. 11753)This invention relates to a method of treating a metal surface inpreparation for applying a corrosion-resistant coating thereto and moreparticularly to a method of treating steel in preparation for glassingand the resultant glass coated article. This application is acontinuationin-part of an application entitled Preparation of Steel forGlassing, Serial N0. 305,072, filed August 18, 1952, now abandoned.

Considerable ditficulty has been encountered in the past when attemptingto coat various types and thicknesses of steel with glass or vitreousenamel. The glass or vitreous enamel coating is subjected to fishscaling and reboiling defects which are attributed to hydrogen gasevolved in the coating application process.

To prevent fish scaling and reboiling resort has been made in the pastto the to bubbly glass and atmosphere firing and to attempt to removeall sources of hydrogen encountered during the glass coating process.

The present invention it is believed takes a novel approach to theproblem of eliminating fish scaling and reboiling in that hydrogen isused to advantage to solve the hydrogen problem that is believed to bethe cause of the fish scaling and reboiling.

In general the invention consists in cathodically bombarding a steelarticle with atomic hydrogen in a bath containing a hydrogen absorptionaccelerating agent to increase the porosity of the steel, and thereafterremoving a substantial portion of the hydrogen from the steel so thatthe hydrogen developed during firing of the glass coating may bereceived and stored within the porous steel to eliminate the build-up ofhydrogen pressure beneath the coating.

The invention has been embodied in a process illustrated in theaccompanying drawings and which constitutes the best mode presentlycontemplated of carrying out the invention.

In the drawings:

Figure 1 is a schematic view showing a steel article immersed in anelectrolytic bath containing the hydro gen absorption acceleratingagent;

Fig. 2 is a schematic view showing the steel article disposed in a hotwater rinse;

Fig. 3 is a schematic view showing the steel article immersed in aneutralizer bath;

Fig. 4 is a reproduction of a reflectoscope pattern showing thereflection of ultrasonic vibrations by the back surface of an untreatedsteel article;

Fig. 5 is a reproduction similar to that of Fig. 4 showing thereflection pattern occurring in a steel article subjected to a hydrogenpenetration treatment for five minutes;

Fig. 6 is a reproduction similar to that of Fig. 5 showing the patternafter a ten minute treatment; and

Fig. 7 is a reproduction similar to Fig. 5 showing the pattern after afifteen minute treatment.

In carrying out the invention the steel is first cleaned to remove allthe mill scale aind foreign material from use of expensive premiumsteels,

2,754,222 Patented July 10, 1956 its surfaces to normally present auniformly clean surface throughout. The cleaning operation can beaccomplished by conventional acid pickling, by sand blasting or by, forexample, cathodic pickling as is disclosed by Staley, Journal ofAmerican Ceramic Society, vol. 9, page 795. In some cases the cathodicpickling and the process of the invention may be carried out in the samebath which contains a hydrogen absorption accelerating agent as will bedescribed hereinafter, although it has been found that hydrogenabsorption accelerating agents adversely affect the removal of certaintypes of scale from the steel.

However, the usual procedure which is applicable to all steels is thatafter the cleaning is completed by whatever method is employed the steelor article to be glass coated is placed in an electrolytic bathcontaining a hydrogen absorption accelerating agent, as shown in Figure1, wherein the steel is employed as the cathode and a generallyinsoluble element, such as graphite or platinum, is made the anode. Thepurpose of this treatment is to bombard the steel cathode with what isbelieved to be atomic and/or ionic hydrogen so as to increase theporosity of the steel by creating, enlarging or blowing up the voids orrifts that are formed or exist in the steel.

The hydrogen absorption accelerating agent is included in the bath toaccelerate the penetration and absorption of the hydrogen atoms in thesteel and takes the form of a compound derived from an element of groupVA or VIA in the periodic table such as arsenic, selenium or antimony.Arsenic pentoxide, AS205, has been shown to be very elfective inpromoting penetration of hydrogen into the steel. The arsenicpentoxideshould not be less than ten parts per million or not less than.00l% of the bath or equivalent molar amounts. However, it has beennoted that the length of time required for the treatment substantiallyincreases and the efficiency of the process decreases as theconcentration of the arsenic pentoxide is lowered below .025% by weightof the bath.

The temperature of the electrolytic bath is preferably maintainedbetween about and 200 F. to achieve the most effective hydrogenpenetration and effective enlargement of the voids.

The electrolyte in the bath may consist of any compound which willdisassociate in water and conduct an electric current and which willgenerate hydrogen at the cathode. Sulphuric acid, sodium hydroxide orsodium sulphate are compounds which may be employed as the electrolyte.It has been found that about a 7% solution, by weight, of sulphuric acidmakes an extremely satisfactory electrolyte, for sulphuric acid isreadily available and the electrolyte may be conveniently regenerated bythe mere addition of water.

The time ofimmersion of the steel Within the bath to obtain thenecessary enlargement of the voids is dependent on the thickness,composition and structure of the steel employed and the nature andoperating conditions of the electrolytic bath.

It is desirable to employ an insoluble anode, such as graphite orplatinum, in the electrolytic circuit so that the anode material willnot be plated on the steel cathode during operation of the electrolyticbath.

When an electric current is applied between the anode and cathode,hydrogen is generated at the cathode, i. e. the article being treated.The nascent hydrogen thus generated is believed to be in the atomicand/0r ionic form, and as such possesses great penetrating power. Thishydrogen penetrates the lattice structure of the steel to expand thesame and is believed to accumulate Within the voids, rifts ordiscontinuities in the steel Where it re combines as hydrogen gas, Hz.

As the generation of hydrogen at the cathode continues,

the pressure of the hydrogen gas accumulated within the voids increasesthe porosity of the steel by what is believed to be a blowing up orenlargement of the voids in the steel.

The term void as used in connection with the metallographic structure ofsteel has never been consistently defined in the metallurgical field,and thus void in the present description and claims is intended to meana degree of porosity in the steel between the limits of an area of lowelectron density to a metallographical visible space in the steel.

The increase of porosity of the treated steel has been proven by severaltesting methods such as density measurements, ultrasonic vibrationreflecn'ons, and metallurgi cal examinations of cross-sections of steel.For example, a density measurement of a given piece of untreated, asreceived, SAE 1015 steel was made and found to be 7.8748 grams per cc.This sample was placed in a 7% H2804 bath having an addition of 0.5 gramper liter of AS205 and at a temperature of 160 F. and subjected to acurrent density of l8 amperes per square foot for a period of 15 minutesto elfect a penetration of hydrogen into the steel. After the hydrogentreatment, the density of this sample was found to be 7.8703 grams percc. thereby indicating a decrease in density or an increase in porosityof the steel due to the hydrogen treatment.

A second procedure employed to show the increase of porosity of thetreated steel was that of ultrasonic testing. The results of these testsare shown in Figs. 4 through 7. Briefly the procedure consists oftransmitting mechanical ultrasonic vibrations through the steel sheet asby a half inch angle, five megacycle crystal. The vibrations arereflected by the opposite or back edge or surface of the steel and thetime interval between the impulse and the reflection is measured by anultrasonic reflectoscope. However, when the steel contains an unusualnumber of discontinuities or voids, the ultrasonic impulses are alsoreflected by the surface surrounding these voids.

As the discontinuities are located between the surface wherein theultrasonic impulse is introduced and the back end surface, the timeinterval between the impulse and the reflection from the surroundingsurfaces of the voids will be less than the time interval for the backsurface reflection of the steel itself and thus the reflection from thediscontinuity will be recorded on the reflectoscope as a pip locatedforwardly of the pip indicating the back surface reflection.

To determine the eifect of hydrogen penetration treatment on thestructure of the steel, several pieces of SAE 1015 steel were sandblasted and introduced as the cathode into an electrolytic bathconsisting of 7% H230. and 0.5 gram per liter of AszOs at a temperatureof160 F. A current density of 18 amperes per square foot was employedand the pieces were subjected to this bath for periods of 5, l0, and 15minutes, respectively. The treated pieces were rinsed in boiling waterfor four minutes and neutralized for one minute in 2 grams per liter ofsodium cyanide at 130 F.

An untreated piece of steel gave the pattern shown in Fig. 4. Thispattern shows one major pip caused by the reflection of the ultrasonicvibration from the back edge or surface of the steel. This pip is knownas a back edge reflection.

The pattern given by the steel which was treated for five minutes isshown in Fig. 5. The small or rounded pips indicate that not all of thevibrations reached the back surface of the steel but that a portion ofthe vibrations instead were reflected by the surfaces surrounding thevoids or discontinuities. The back edge reflection pip indicates thatthe back edge reflection of the steel is considerably smaller than thecorresponding back surface reflection of the untreated steel shown inFig.4. 7

After treating the specimen for minutes the pattern, -as seen in Fig. 6,shows that the back edge-reflection, is further reduced in size andindicates, by the increased size of the forward rounded pips, that thenumber and size of the voids or discontinuities have increased.

The pattern shown in Fig. 7, which represents a hydrogen treatment of 15minutes, indicates that the discontinuities have increased in size tosuch a degree that the vibrations did not penetrate to the back edge ofthe steel and thus the large pip shown inFigs. 4, 5 and 6, indicatingthe back edge reflection, is no longer present. In this pattern theentire vibration is reflected by the voids or discontinuities adjacentto the front surface of the steel.

From these ultrasonic tests it is clearly indicated that the porosity ofthe steel is increased by the hydrogen penetration operation, or theexisting discontiuities or voids are enlarged or new discontinuities areformed by the action of the hydrogen in the steel.

After the hydrogen bombardment is completed, the hydrogen gasaccumulated'within the steel. during the electrolytic action should beremoved without any considerable reduction of the size of the voidssince if the hydrogen is not removed, the hydrogen would pass from thesteel during firing of the glass coating that is to be fused thereon ashereinafter described, and cause excessive boiling in the glass. Undernormal conditions the hydrogen will pass freely out of the steel to theatmosphere. However, this is a very lengthy process and the hydrogen isordinarily removed by boiling water or other heat treatments.

Rinsing of the steel in boiling water is illustrated in Fig. 2 where thehot water bath there shown serves to remove hydrogen from the voids andrinses the excess electrolyte from the surface of the steel.

On completion of the Water rinse when an acid is employed as theelectrolyte, the steel is then placed in a neutralizing bath, shown inFig. 3. The neutralizing bath may be made by the addition of smallamounts of borax, sodium hydroxide or sodium cyanide to water. Thetemperature of the neutralizing bath is maintained from about F. toabout 200 F. It is necessary to remove all traces of the acid so as toprevent corrosion of the steel before the same is glass coated.Corrosion by the acid occurs Very rapidly so it is desirable tocompletely remove all traces of the acid from the surface of the steel.

With the hydrogen gas substantially removed from the steel, the steelmay then be coated with a glass or vitreous enamel composition by any ofthe conventional processes. After coating, the steel is fired at anelevated temperature of about 1200 F. to 1600 F. to fuse the glasscoating to the steel. The particular firing temperature within the 1200F. to 1600 F. range depends on the sag resistance of the steel to whichthe glass is applied.

During the firing, water from a number of previously mentioned sourcesreacts with the iron to form atomic hydrogen which penetrates the steeland is dissolved there- In. a

After firing, as the steel is cooled, the dissolved atomic hydrogen,which is more soluble in steel at high temperatures than at lowtemperatures, tends to pass out of solution from hydrogen within thevoids. As the voids are believed to have been substantially enlarged bythe electrolytic hydrogen penetration, the hydrogen gas which collectswithin the enlarged voids has ample room into which to expand andtherefore no damaging pressure is exerted by the gas beneath thehardened glass coating. As there is no excessive pressure exerted by thehydrogen gas beneath the coating, there is no tendency for the coatingto pop off as fish scales or to reboil on subsequent firing.

With the use of the present invention premium steel and bubbly glassstructures are not required because of the increase in the porosity ofthe steel by hydrogen penetration. Therefore hot rolled, cold rolled,killed, semikilled, and rimmed steel and steels of considerable rangethe steel and accumulate as molecular all successfully be glass coatedwith the present process. This factor enables the cheaper non-premiumsteels to be coated. With the present invention, steel having increasedstrength and improved physical properties may be coated and this opensup the glass coating field to a wide variety of products whichheretofore had not been susceptible to coating because of therestrictive nature of the steel to which a coating could be successfullyapplied. For example, heretofore enameling iron, which is asubstantially pure iron having a carbon content of .0l% to .03%, hasbeen generally exclusively employed as the base for glass coatingsbecause the higher carbon steels cause fish scaling and reboiling of thecoating. However, with the use of the present invention higher carbonsteels can be successfully glass coated Without evidence of fish scalingor reboiling defects in the coating. Not only are the higher carbonsteels considerably less expensive than the enameling iron, but thehigher carbon steels generally have increased physical properties Whichincrease the variety of products which can be enameled or glass coated.

Since the process of the invention eliminates the need for hydrogendefect control in the design of the glass coating composition it makespossible the use of a nonbubble type of glass for a ground coat on thesteel. In addition, as there are no appreciable bubbles or voids in thesubstantially non-bubbly glass, a thinner coating may be employed, ascompared to the bubbly type, Without reducing the effective corrosionresistance of the coating.

Various modes of carrying out the invention are contemplated as withinthe scope of the following claims particularly pointing out anddistinctly claiming the subject matter which is regarded as theinvention.

We claim:

1. In a method of coating steel for glassing, said steel being initiallycoated with a non-uniform coating of mill scale and foreign matter, thesteps comprising cleaning the steel to be glassed to provide a uniformlyclean surface on the steel, making the steel the cathode in anelectrolytic circuit established in a bath containing arsenic pentoxidein an amount not less than .001% of the bath, applying an electriccurrent to said circuit to generate hydrogen at said cathode, saidhydrogen penetrating the cleaned surface of the steel uniformlythroughout under the action of the electric current and the arsenicpentoxide operating as a hydrogen absorption accelerating agent andentering the voids in said steel and exerting pressure Within the voidsto increase the size of the same, removing said steel from the circuit,heating the steel to substantially completely remove the hydrogen fromthe steel, thereafter subjecting said steel to a neutralizing bathmaintained at a temperature from about 12 F. to 200 F. to remove anytraces of acid remaining on the steel, and then applying a glasscomposition to the steel and fusing said composition thereto, thehydrogen subsequently formed during fusion being free during cooling topass into said voids and eliminate the build-up of excess pressurebeneath the glass coating.

2. In a method of glass coating steel, the steps comprisingelectrolytically penetrating hydrogen into the steel in a bathcontaining a hydrogen absorption accelerating agent to substantiallyaccelerate absorption of hydrogen by the steel above the rate ofhydrogen penetration encountered in an electrolytic bath under cathodicpickling conditions and permanently increase the porosity thereof asevidenced by the change in back edge reflections of ultrasonicvibrations transmitted through the steel by a halfinch angle fivemegacycle crystal, removing substantially completely all of the hydrogenfrom the steel, applying a non-bubbly type coating of a glasscomposition to the steel, and firing the coated steel at an elevatedtemperature to fuse the coating to the steel with the hydrogen formedduring firing being free to expand into the porous steel during coolingand eliminate fish scaling of the coating.

of carbon content may 3. In a process for coating steel with a glasscomposition fused to the steel by firing the glass at an elevatedtemperature, the steps comprising subjecting the steel that is to beglass coated to an electrolytic bath containing a hydrogen absorptionaccelerating compound of a metal selected from the group consisting ofarsenic, selenium and antimony with the amount of said metal in the bathbeing equivalent in action to not less than the amount of arseniccontained in a .001% by Weight of a solution of arsenic pentoxide and ina maximum concentration preventing development of defects in the glasscoating fused to the steel, penetrating hydrogen into said steel by aneX- ternal current applied to said bath with said hydrogen penetrationbeing accelerated by said compound and continuing the hydrogen treatmentuntil the lattice structure of the steel is increased and the densitythereof is decreased, thereafter removing hydrogen from the steel toreduce the hydrogen pressure in the steel below the pressure of thehydrogen subsequently formed at the glass coated surface of the steelduring firing of the glass to enable the subsequently formed hydrogen toexpand into said steel during cooling, coating the steel With the glasscomposition, and firing the steel at an elevated temperature to fuse theglass to the steel with the hydrogen produced by the chemical reactionsduring firing passing into the lattice structure of the steel uponcooling of the steel to thereby prevent the build-up of hydrogenpressure beneath the coating.

4. In a method of coating steel by a glass composition fused to thesteel by firing the glass at an elevated temperature, the stepscomprising locating the steel in an electrolytic bath containing arsenicpentoxide in an amount not less than .0()1% by Weight of the bath and ina maximum concentration preventing development of defects in the glassfused to the steel, making the steel the cathode in an electrolyticcircuit, said circuit having a substantially inert anode and anelectrolyte With said arsenic pentoxide increasing the rate ofabsorption of said hydrogen in the steel, applying an electric currentto said circuit to generate hydrogen at the cathode With the hydrogenpenetrating into the steel to increase the porosity thereof, removingsaid steel from the circuit, applying heat to said steel to remove asufiicient amount of the hydrogen from the steel to reduce the hydrogenpressure in the steel below the pressure of the hydrogen subsequentlyformed at the glass coated surface of the steel during firing of theglass to enable the subsequently formed hydrogen to more readily expandinto said steel during cooling, coating said steel with the glasscomposition, and firing the steel at an elevated temperature to fuse theglass to the steel with the glass coating.

5. in a method of coating steel With a glass composition fused to thesteel by fusing the glass at an elevated temperature, the stepscomprising subjecting the steel that is to be glass coated to anelectrolytic bath containing a hydrogen absorption accelerating elementselected from the group consisting of Group VA and Group VIA of thePeriodic Table of Elements with the amount of said element in the bathbeing equivalent in action to not less than the amount of arseniccontained in a .001% by weight of a solution of arsenic pentoxide and ina maximum concentration preventing development of defects in the glassfused to the steel, penetrating hydrogen into said steel by an externalcurrent applied to said bath with said hydrogen penetration beingaccelerated by said element and continuing the hydrogen treatment untilthe lattice structure of the steel is increased and enlarged and thedensity of the steel is decreased, thereafter removing hydrogen from thesteel to reduce the hydrogen pressure in the steel below the pressure ofthe hydrogen subsequently formed at the glass coated surface of thesteel during firing of the glass to enable the subsequently formedhydrogen to expand into said steel during cooling, coating the steelwith the glass composition, and firing the steel at an elevatedtemperature to fuse the glass to the steel with the hydrogen produced bythe chemical reactions during firing passing into the lattice structureof the steel upon cooling of the steel to thereby prevent the build-upof hydrogen pressure beneath the coating.

6. In a method of prising penetrating hydrogen into suificient topermanently increase the porosity of the steel as evidenced by thechange in back edge reflections of ultrasonic vibrations transmittedthrough the steel by a half-inch angle five megacycle crystal, removingthe hydrogen from the steel, applying a coating of a non-bubbly glasscomposition to the steel, and firing the coated steel at an elevatedtemperature to fuse the coating to the steel with the hydrogen formedduring firing being free to expand into the porous steel during coolingand eliminate fish scaling of the coating.

glass coating steel, the steps comthe steel in an amount 7. A steelarticle of manufacture having an outer surface finished to be coveredw'th a glass coating, substantially the entire area of the steel of saidarticle underneath said finished surface having interspaced voids of asize and number of interfere with the normal back edge reflection of thearticle by ultrasonic vibrations, and a non-bubbly type glass coatingfused to the finished surface of the article, said voids receiving andstoring hydrogen generated during the fusing of the glass to the steelto eliminate fish scaling and reboiling of the glass by said hydro.

gen.

References Cited in the file of this patent Stayley: American CeramicSoc. Ioun, vol. 9 (1926), page 795.

Journal of the Iron and Steel Institute, vol. 147 (1943), page 139?,paper by Stuart et a1.

1. IN A METHOD OF COATING STEEL FOR GLASSING, SAID STEEL BEING INITIALLYCOATED WITH A NON-UNIFORM COATING OF MILL SCALE AND FOREIGN MATTER, THESTEPS COMPRISING CLEANING THE STEEL TO BE GLASSED TO PROVIDE A UNIFORMLYCLEAN SURFACE ON THE STEEL, MAKING THE STEEL THE CATHODE IN ANELECTROLYTIC CIRCUIT ESTABLISHED IN A BATH CONTAINING ARSENIC PENTOXIDEIN AN AMOUNT NOT LESS THAN .001% TO THE BATH APPLYING AN ELECTRICCURRENT TO SAID CIRCUIT TO GENERATE HYDROGEN AT SAID CATHODE, SAIDHYDROGEN PENETRATING THE CLEANED SURFACE OF THE STEEL UNIFORMLYTHROUGHOUT UNDER THE ACTION OF THE ELECTRIC CURRENT AND THE ARSENICPENTOXIDE OPERATING AS A HYDROGEN ABSORPTION ACCELERATING AGENT ANDENTERING THE VOIDS IN SAID STEEL AND EXERTING PRESSURE WITHIN THE VOIDSTO INCREASE THE SIZE OF THE SAME, REMOVING SAID STEEL FROM THE CIRCUIT,HEATING THE STEEL TO SUBSTANTIALLY COMPLETELY REMOVE THE HYDROGEN FROMTHE STEEL, THEREAFTER SUBJECTING SAID STEEL TO A NEUTRALIZING BATHMAINTAINED AT A TEMPERATURE FROM ABOUT 120* F. TO 200* F. TO REMOVE ANYTRACES OF ACID REMAINING ON THE STEEL, AND THEN APPLYING A GLASSCOMPOSITION TO THE STEEL AND FUSING SAID COMPOSITION THERETO, THEHYDROGEN SUBSEQUENTLY FORMED DURING FUSION BEING FREE DURING COOLING TOPASS INTO SAID VOIDS AND ELIMINATE THE BUILD-UP OF EXCESS PRESSUREBENEATH THE GLASS COATING.